My favorite hook for formal repositories is the one that refuses multiple heads. It s great when you ve got a continuous integration system that needs a post-merge tip to build automatically.

A few examples are here: MercurialWiki: TipsAndTricks - prevent a push that would create multiple heads

I use this version from Netbeans:

# This software may be used and distributed according to the terms
# of the GNU General Public License, incorporated herein by reference.
#
# To forbid pushes which creates two or more headss
#
# [hooks]
# pretxnchangegroup.forbid_2heads = python:forbid2_head.forbid_2heads

from mercurial import ui
from mercurial.i18n import gettext as _

def forbid_2heads(ui, repo, hooktype, node, **kwargs):
    if len(repo.heads()) > 1:
        ui.warn(_( Trying to push more than one head, try run "hg merge" before it.
 ))
        return True

I ve just created a small pretxncommit hook that checks for tabs and trailing whitespace and reports it rather nicely to the user. It also provides a command for cleaning up those files (or all files).

See the CheckFiles extension.

Another good hook is this one. It allows multiple heads, but only if they are in different branches.

Single head per branch

def hook(ui, repo, **kwargs):
    for b in repo.branchtags():
        if len(repo.branchheads(b)) > 1:
            print "Two heads detected on branch  %s " % b
            print "Only one head per branch is allowed!"
            return 1
    return 0

I like the Single Head Per Branch hook mentioned above; however, branchtags() should be replaced with branchmap() since branchtags() is no longer available. (I couldn t comment on that one so I stuck it down here).

I also like the hook from https://bobhood.wordpress.com/2012/12/14/branch-freezing-with-mercurial/ for Frozen Branches. You add a section in your hgrc like this:

[frozen_branches]
freeze_list = BranchFoo, BranchBar

and add the hook:

def frozenbranches(ui, repo, **kwargs):
    hooktype = kwargs[ hooktype ]
    if hooktype !=  pretxnchangegroup :
        ui.warn( frozenbranches: Only "pretxnchangegroup" hooks are supported by this hook
 )
        return True
    frozen_list = ui.configlist( frozen_branches ,  freeze_list )
    if frozen_list is None:
        # no frozen branches listed; allow all changes
        return False
    try:
        ctx = repo[kwargs[ node ]]
        start = ctx.rev()
        end = len(repo)

        for rev in xrange(start, end):
            node = repo[rev]
            branch = node.branch()
            if branch in frozen_list:
                ui.warn("abort: %d:%s includes modifications to frozen branch:  %s !
" % (rev, node.hex()[:12], branch))
                # reject the entire changegroup
                return True
    except:
        e = sys.exc_info()[0]
        ui.warn("
ERROR !!!
%s" % e)
        return True

    # allow the changegroup
    return False

If anyone attempts to update the frozen branches (e.g., BranchFoo, BranchBar) the transaction will be aborted.